1. Field of the Invention
The present invention pertains to pumping apparatus. More specifically, the present invention pertains to reciprocating pumps of the type used for producing fluids from subsurface wells.
2. Description of the Prior Art
Subsurface wells, particularly those for producing underground hydrocarbon fluids, typically include a string of tubing or xe2x80x9cproduction stringxe2x80x9d which extends from near the bottom of the well to the surface for flow of fluids through a flow line connected to the production string at the surface. For wells which do not have enough pressure to produce fluids on their own, some type of pumping system must be employed.
Pumps for lifting subsurface fluids to the surface of an oil well have been in existence for many years. One type of pump, typically referred to as a lift pump, usually includes a tubular barrel (which may be a portion of the production tubing) and a cooperating plunger assembly which reciprocates therein. The plunger assembly may be attached to a rod or string of rods which extends to the surface of the well and by which the plunger assembly may be reciprocated by a source of power such as an internal combustion engine or an electrical motor. Examples of such pumps may be seen in U.S. Pat. Nos. 4,691,735 and 5,178,184.
A lift pump typically includes a standing valve which is fixed relative to the pump barrel and a traveling valve which is a component of the plunger assembly. The standing valve and traveling valve act as check valves, opening and closing, opposite each other, on upstrokes and downstrokes of the plunger assembly. For example, as the plunger assembly and the attached traveling valve are lowered on a downstroke, the standing valve is closed, blocking reverse fluid flow therethrough, and the traveling valve is opened, allowing fluid within the pump barrel to be displaced through the traveling valve into the production tubing thereabove. On the subsequent upstroke, the traveling valve closes, lifting the column of fluids thereabove towards the surface. Since pressure in the pump barrel below the traveling valve decreases during the upstroke, the standing valve then opens allowing fluid to flow into the pump barrel from the formation for a succeeding downstroke. As this process continues, fluid flows through the standing valve and into the pump barrel during the upstrokes and fluid above the traveling valve is lifted toward the surface of the well on the upstroke.
Since the fluids being produced from a well are typically located at some distance below the surface, in most cases hundreds of thousands of feet, a power unit associated with a lift pump must lift: i) a long string of steel rods, ii) the plunger assembly, and iii) a column of fluid the length of which is approximately equal to the depth of the well. This requires a great deal of energy. With lift pumps of the prior art, no fluid production occurs on the downstroke in which the plunger assembly and the string of rods is lowered before another upstroke. Thus the lifting of great weights on the upstroke requires a great amount of energy while the energy from the weight of the rods and plunger assembly on the downstroke is wasted and not utilized.
The great difference between the load on an upstroke and the lack of load on a downstroke creates a counterbalance problem on the power unit. This great difference in weight cannot be fully counterbalanced. If the power unit is powered by electricity (an electric motor), the electric motor draws much higher amperage on the upstroke than on a downstroke.
Another problem with lift pumps of the prior art is associated with stretching of the rods by which the plunger and fluids are lifted in the production string. The rods stretch on the upstroke and relax on the downstroke. This results in loss of movement or plunger travel as compared to the length of movement of the power unit stroke. This results in inefficiency.
Attempts have been made in the prior art to reduce the load and the energy required to lift fluids to the surface of a well. Specifically, attempts have been made to utilize the energy normally lost during the downstroke of the plunger assembly by pumping on the downstroke. One such attempt is described in U.S. Pat. No. 5,314,025. Although this pumping apparatus appears to utilize the weight of the rods in response to gravity as a source of pumping energy for pumping on the downstroke, it does not substantially reduce the energy required on the upstroke. This particular apparatus has other characteristics which have apparently prevented it from being accepted in the industry.
The present invention comprises pumping apparatus for pumping fluids through the production string of a subsurface well. The apparatus, in a preferred embodiment, comprises a fixed tubular pump barrel attached to the lower end of the production string which has a standing valve in the lower end thereof to permit flow of fluids into the barrel but preventing flow of fluids out of the barrel. It also includes a reciprocating tubular pump plunger concentrically disposed in the pump barrel for sliding and sealing reciprocal movement therein. The reciprocating plunger is provided, on its bottom end, with a traveling valve which permits flow of fluids from the fixed barrel through the reciprocating plunger but prevents flow of fluids through the reciprocating plunger into the fixed pump barrel. The upper end of the reciprocating plunger projects out of the fixed barrel into the production string and is provided with flow passages at the upper end thereof through which fluid may flow from the barrel, through the plunger into the production string. The lower end of a string of rods is attached to the reciprocating pump plunger and extends upwardly through the production string to near the surface. A cylindrical polished rod is connected to the top of the string of rods and extends upwardly through the sealing means in the wellhead for sliding and sealing reciprocation therethrough. A power unit is operatively connected to the polished rod for lifting and lowering the polished rod and the string of rods to lift and lower the reciprocating pump plunger within the fixed pump barrel.
Unique features of the pumping apparatus of the present invention reside in the fact that the upper end of the reciprocating plunger projects out of the barrel and in the fact that the polished rod is of a diameter at least as great as the outside of the diameter of the pump plunger. For this reason, the volume of the polished rod displaced from the production string on the upstroke is at least as great as the volume of fluids displaced by the reciprocating plunger on the upstroke. Thus, no fluids are displaced or will flow through the production tubing into the flow line on the upstroke and the only energy required during the upstroke is energy required to lift: (a) the reciprocating pump plunger and (b) the string of rods attached thereto. However, as the string of rods and the reciprocating pump plunger are lowered on the downstroke, the energy derived form the weight of the string of rods and the pump plunger, due to the gravitational pull thereon, is utilized to force fluids in the fixed pump barrel through the reciprocating pump plunger and its traveling valve and through the production string to the surface for flow through the flow line connected to the production string. In summary, production is exactly the opposite of the typical lift pump in which fluids are produced on the upstroke; that is all of the production of fluids occur on the downstroke.
One of the major advantages of the pumping apparatus of the present invention is the utilization of the normally wasted energy associated with downward movement of the reciprocating pump plunger and the string of rods attached to the reciprocating pump plunger to force fluids to the surface of the well during the downstroke and the fact that the only energy required during the upstroke is energy required for lifting the string of rods and the reciprocating pump plunger. If the power unit is powered by an electrical motor, the motor draws essentially the same amperage on the upstroke as the downstroke, resulting in an approximately 50% reduction in electrical cost per barrel of produced fluid.
There is much less wear and tear, requiring less maintenance, yet the equipment is no more complicated and no more expensive than prior art lift pumps. Many other objects and advantages of the invention will be apparent from reading the description which follows in conjunction with the accompanying drawings.